My sister, my rival

Don't go into a severe state of shock just yet, but it seems that all is not well with the Collinses. Joan, Hollywood legend and weddings specialist, has finally admitted that she has fallen out badly with her sister Jackie, purveyor of fluorescent-jacketed tales of large cars and larger egos to the airport bookstores of the world. On next Sunday's South Bank Show, the 66-year-old actress confesses to a recent history of squabbles and silences behind the public facade of artificial smiles.

'You can choose your friends but you can't choose your family,' she grumbles. 'We drifted apart. I love my sister, but I'm not as close to her as I used to be.'

Something about this revelation feels deeply right. It would have been counterintuitive had Joan confided that she likes nothing better than to curl up on the sofa with the author of such works as The Bitch, The Stud and Lady Boss for a game of Connect 4. Yet it appears that their rivalry is a recent phenomenon. It stems, Joan reckons, from her own forays into fiction, which began in the late 1980s with the publication of the now out-of-print romance Prime Time. 'I don't think she was thrilled when I started writing,' Collins tells Bragg.

Doubtless many people had a similarly dismayed reaction to this literary debut (indeed her publisher, Random House, later tried to sue her for delivering a 'fragmented' manuscript after being paid an advance), but not, it's reasonable to suppose, as acute as Jackie's. For Joan has committed that particularly terrible crime: she has trespassed onto territory staked out by a sibling as her own.

One person who isn't surprised by the news is Professor Virginia Molgaard, a psychologist at Iowa State university who specialises in the study of sibling rivalry. 'We often find that children select different activities to excel in, in order to avoid comparisons,' she says. 'But when two children try to be good at the same thing, there are apt to be comparative evaluations. That often leads to oneupmanship, or putting people down.'

More often than not, though, public spats are avoided with some tried-and-tested ploys. Staking out radically different regions of the same territory is a policy that seems to work - and one used by the brothers Charles and Jonathan Powell. Charles was a longtime foreign policy aide to Margaret Thatcher; Jonathan, an aide to Tony Blair, has been nicknamed 'the real deputy prime minister' in Westminster circles. One friend was quoted as saying that 'publicly, they pretend there are some differences, when privately they get on perfectly well most of the time'.

Getting the hell out is another effective method. This newspaper recently reported the rise of Wendy Alexander and her brother Douglas, Scottish minister in charge of Labour's campaign to eradicate policy in Scotland and former speechwriter to Donald Dewar respectively. Douglas has been named a senior manager of Labour's next general election campaign - but Wendy has made clear her intentions to stay in Scotland, where she is tipped as a future foreign minister.

The most comprehensive attempt at boundary-setting, of course, has been pulled off by Christopher Hitchens, the Washington-based socialist firebrand, and his Blair-baiting brother Peter, an Express writer. Christopher thinks Peter is a 'reactionary'; Peter says Christopher is a 'destructive radical'. Neither seem to suffer much damage to their self-esteem - although, in a recent interview, Peter did hint at something less than healthy in their relationship.

'Being Peter Hitchens is about not being Christopher Hitchens, to some extent, just as Canada is about not being the United States,' he observed. 'But it doesn't mean that I spend my whole time thinking ...how can I differentiate myself from him?'

An option to be avoided is what's known in the psychological literature as the Kylie and Dannii approach, characterised by a desperate and unconvincing attempt to seem like the best of friends in public when the underlying rivalry is embarrassingly clear to all.

'Where there is a continuing rivalry into adulthood, people do often tend just to withdraw from one another,' Professor Molgaard notes. 'One road is to resolve the rivalry; another is to stay conflicted, but the easiest is just to say: I don't want to hear you bragging - so I'm not going to talk to you.'

And, in the end, isn't there something sinister about siblings who seem to relish sharing a career? The Brontës, huddling in their cottage, driving each other slowly mad with backhanded compliments? Or the Osmonds, all happy-families in public and dark depressions in private? Or - and here the evidence speaks for itself - The Proclaimers? Joan Collins is right to tell the truth about her sisterly dispute. The alternatives look a lot worse.

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